De-orbiting or re-orbiting artificial satellites at the end of their operational lifetime is an operation that is tending to be applied systematically, for two major reasons:                with geostationary satellites, the geostationary orbit can receive only a limited number of satellites occupying positions that are predefined and allocated by the International Telecommunication Union (ITU), and it is desirable or even necessary to release those positions at the end of the working lifetime of a satellite so that these positions can be reused;        for satellites in low orbit, the number of satellites is increasing and in the shorter or longer term they present risks of colliding with one another or, which would be much more serious, with an inhabited space station, and rules are currently being drawn up for limiting the orbiting lifetime of such satellites once they have reached the end of their mission, with one recommendation that is widely accepted at present being a lifetime limited to 25 years.        
A satellite is generally de-orbited or re-orbited by using thrust from at least one of two nozzles or pluralities of nozzles that were previously used for controlling the orbit and/or the attitude of the satellite while it was in operation, after which the residual quantity of propellant, or a fraction of said residual quantity, is used to feed said nozzles or pluralities of nozzles so as to perform said de-orbiting or re-orbiting maneuver.
For a commercial satellite, and in particular for a telecommunications satellite in geostationary orbit, the advantage in prolonging the operating period of the satellite as much as possible can be seen immediately. The operator has every interest in triggering the change-of-orbit sequence as late as possible, while still remaining compatible with the minimum requirements in terms of propellant needed for feeding the nozzles or pluralities of nozzles so as to ensure the satellite is put onto a “graveyard” orbit.